1. Field of the Invention
This invention relates in general to a method and apparatus for tracking a particle and in particular to a method and apparatus for tracking a particle in order to analyze fluid flow in industrial vessels.
2. Background Information
There are many industrial processes (for example, in the chemical industry), in which a solid or fluid is added to a fluid in a vessel. The contents of the vessel may be mixed either during or after the addition. It is often desirable that the resulting mixture be uniform or homogeneous. The quality of mixing can have a profound influence on the quality of the resulting product. In industrial vessels, mixing equipment acts upon the large scale fluid convection patterns, resulting in `macromixing`. However, the fluid flow is a complex three-dimensional and time-dependent phenomenon. Experimental characterization of such flows has long been considered a difficult challenge. One class of methods for analyzing these large scale fluid patterns, and the mixing process, involves the addition of a `tracer` that can be tracked as it moves under the influence of fluid forces.
Methods that employ more than one particle frequently use particles ranging in size from approximately a micrometer or less to several millimeters and consisting of insoluble solids, liquids, or gases. These methods usually require optical methods, such as photography, to observe the motion of the particles. Optical methods are difficult to use in opaque vessels or processes that require the absence of light. If the fluid contains other suspended particles, or is opaque, then methods based on the optical detection of many particles are not practical. It is also difficult to follow a single particle, as it moves through the vessel, over an extended period of time.
A second class of methods for analyzing fluid flow and mixing processes uses a single tracer particle, which is tracked over an extended period of time. The tracer particle can be either `active` or `passive`. An example of a passive tracer particle is given in a paper by D. F. Scofield and C. J. Martin titled "Mixing Time Distributions and Period Doubling in Stirred Tanks" presented at the 1990 Annual Meeting of the American Institute of Chemical Engineers (Chicago, November 1990). They described a system for measuring the "time-dependent mixing in a common 284 liter (75 gallon) industrial mixer . . . using a Lagrangian Marker Particle (LMP) method". Their tracer particle was a one centimeter diameter, neutrally buoyant sphere, in a transparent tank. They used video equipment to capture electronic images of the particle, from which the particle position could be computed. The particle could be tracked for hours. By inferring the convective flow field from the trajectory of the particle, they were able to determine "the time dependent structure of the flow". The method requires that the particle be illuminated. Although most of the hardware is commercially available, the method uses computationally intensive image analysis methods. The method described by Scofield and Martin is an example of an optical imaging technique. If the imaging signal has frequencies in the radio frequency range, it is a method similar to radar. If the imaging signal is an acoustic wave (compressive wave), then it is a method similar to sonar. In all of these methods, the tracer particle is a passive object that reflects an incident signal to one or more detectors.
The use of a more complex active particle can minimize the complexity of the detection equipment. Jan van Barneveld, Willem Smit, Nico M. G. Oosterhuis, and Hans J. Pragt in an article titled "Measuring the Liquid Circulation Time in a Large Gas-Liquid Contactor by Means of a Radio Pill. 1. Flow Pattern and Mean Circulation Time" (Industrial and Engineering Chemistry Research, Volume 26, pages 2185 to 2192, November 1987) describe a neutrally buoyant "pill" three centimeters in diameter. The tracer particle emits a radio frequency signal of approximately 1 MHz. Aerials are mounted inside the vessel in the vicinity of the mixing impeller. As the particle passes through the opening in the aerial, its signal is detected and logged. J. C. Middleton has described a similar system, in "Measurement of circulation Within Large Mixing Vessels", Third European Conference on Mixing, Apr. 4 to 6, 1979, paper A2.
Instead of using a tracer particle containing a radio frequency source, one could use a particle containing a magnet, and detect its passage through an aerial by the induced electromagnetic signal. The elapsed time between repeated passages of the particle through the aerial are used in the calculations of a Circulation Time Distribution, which in turn is a measure of the macromixing in the vessel. These methods, whether using a radio transmitter or a magnetic particle, are limited to the detection of a particle as it passes through a plane surface defined by the aerial. The presence of the aerial may influence the flow field. If the detectors were capable of determining direction, then three or more detectors could be used to determine the position of a particle transmitting a signal. Many range finding and navigation systems use these principles.
A system of tracking in which the tracer particle contains an acoustic transmitter, but no receiver, is possible. If the signal consists of a series of pulses, the transmission rate can be synchronized with a clock in the signal processing equipment. From this, the initial time of the pulse transmission is known, and the position of the tracer particle can be determined by triangulation. In this system, it is necessary that the transmitter clock on the particle and the clocks on the detectors remain synchronized. Such a system for determining the position of a torpedo is described in U.S. Pat. No. 3,205,475 issued to Rene N. Foss. The present invention is superior in the sense that this synchronization is not necessary.